1. Field of the Invention
The present invention relates to a braking control system for controlling a braking force applied to each of wheels of an automotive vehicle, and more particularly to the braking control system which holds the vehicle in its stopped state, when the vehicle stops on a steep slope.
2. Description of the Related Arts
In general, when a moving vehicle is braked, axle loads applied to the front and rear portions of the vehicle respectively will be different from each other due to the moving load caused by the braking operation. Therefore, the braking force applied to front wheels and the braking force applied to rear wheels for locking all of the wheels simultaneously are not in direct proportion to each other,, but in such a relationship as called an ideal braking force distribution, which may be varied depending upon the presence or absence of the load on the vehicle. If the braking force applied to the rear wheels exceeds the braking force applied to the front wheels, the directional stability of the vehicle will be deteriorated. In order to keep the braking force applied to the rear wheel lower than that applied to the front wheel and provide a distribution in close proximity to the ideal braking force distribution, a proportioning valve is provided between the rear wheel brake cylinders and a master cylinder. According to this arrangement, the distribution characteristic has a pair of lines having different gradients from each other. When the difference of the loads applied to the inner and outer wheels of a turning vehicle is taken into consideration for example, it is necessary to reduce the braking force applied to the rear wheels much lower than the braking force applied to the front wheels. Therefore, in the case where it is so arranged that the hydraulic braking pressure is always restricted by the proportioning valve, the braking force distributed to the rear wheels will be reduced, so that a large depressing force to a brake pedal will be needed to ensure a desired deceleration, and an excessive load will be applied to the front wheels.
In order to solve the problem as described above, U.S. Pat. No. 5,624,164 discloses a braking force distribution control system, which is adapted to control the braking force applied to the rear wheel in a predetermined relationship with the braking force applied to the front wheel, by controlling pressure control valves disposed between the wheel brake cylinder and a hydraulic booster or the master cylinder, and controlling a changeover valve disposed upstream of the control valves.
An ordinary passenger vehicle has a pair of wheels at each of its front and rear sides. Either the front wheels or the rear wheels of that vehicle are operatively connected with an engine to be driven directly thereby, while the rest of the wheels are not connected with the engine so as to be served as non-driven wheels. A vehicle having the driven wheels at its front side is called a front drive vehicle, while a vehicle having the driven wheels at its rear side is called a rear drive vehicle. And, a vehicle having the driven wheels at both of the front and rear sides is called a four-wheel drive (4WD) vehicle. As for a driving system of the four-wheel drive vehicle, various types of the system are known, such as a part time system, full time system, and the like. According to the full time system, all of the front and rear wheels are connected through a front differential gear, a rear differential gear, and a center differential gear.
According to the above-described part time system of the four-wheel drive vehicle, when the vehicle turns with its four wheels driven, its cornering maneuver is difficult due to a rotational difference between the front and rear wheels. This is called a phenomenon of tight corner braking. According to the full time system of the four-wheel drive vehicle, the driving force transmitted to the wheels through a transmission is effectively distributed to the front and rear wheels by the center differential gear, and the rotational difference between the front and rear wheels is compensated, so that a smooth cornering maneuver can be ensured. However, the center differential gear will cause another problem. That is, if one of the front and rear wheels slips to rotate freely, the driving force will not be transmitted to the rest of the wheels at all. In order to avoid this, a center differential locking mechanism for locking the center differential gear manually has been introduced.
Thus, from the view point of the braking force distribution, the hydraulic braking pressure in the rear wheel brake cylinder is restricted by the proportioning valve, or it is controlled by the pressure control valves in accordance with the predetermined relationship in the apparatus as disclosed in U.S. Pat. No. 5,624,164. According to the vehicle having the braking force distribution system as described above, when the vehicle i; moving upward on a steep slope, the actual braking force distribution will be switched from the normal braking force distribution to the one, in which the load to the rear wheels located downward will be larger than the load to the front wheels. In this case, however, if the vehicle is braked, the braking force distributed to the rear wheels will be reduced by the above-described distribution system. As a result, when the vehicle moving upward on the steep slope is to be stopped, it will be caused that the wheel speed of the rear wheel will not become zero, while the wheel speed of the front wheel will become zero. Especially on a rough and steep slope, the vehicle is hardly held to be in its stopped state at a desired position. Also, in the case where the vehicle is moving downward on the steep slope, with its rear end positioned downward, i.e., moving in the reverse direction, and then stopped on the steep slope, the situation will be similar to the one in the above case. In these cases, the depressing force may be increased to apply the sufficient braking force to the rear wheels. Depending upon the gradient of the steep slope, however, a quite large depressing force will be needed, so that a large load will be applied to the braking apparatus for the front wheels which are already held to be in the stopped state.
According to the apparatus as disclosed in U.S. Pat. No. 5,624,164, the hydraulic braking pressure in the rear wheel brake cylinders can be regulated to provide the predetermined relationship by controlling the pressure control valves. However, the apparatus is basically the one for restricting the hydraulic braking pressure in the rear wheel brake cylinders, so that a specific countermeasure will be needed in such an unusual situation that the vehicle is stopped when it is moving upward on the steep slope, or stopped when it is moving backward on the steep slope with its rear end positioned downward. Furthermore, in the case where the vehicle is restarted after it was stopped on the rough and steep slope for example, if the braking force is released suddenly, the motion of the vehicle will become unstable. In this case, therefore, the braking force will have to be released gradually, to restart the vehicle's movement slowly.
According to the full time system of the four-wheel drive vehicle, however, if the center differential gear is locked by the center differential locking mechanism, the phenomenon of tight corner braking will be caused, as described before. As a result, the vehicle's cornering maneuver will be difficult. As a countermeasure against this, simply removing the center differential locking mechanism will need another countermeasure, when one of the wheels will slip to rotate freely, as described before. Furthermore, if the center differential locking mechanism is removed, a further countermeasure will be needed in such conditions as follows. That is, in the case where the vehicle is stopped when it is moving upward on the steep slope, or stopped when it is moving backward on the steep slope, with its rear end positioned downward, even if the vehicle is of the four-wheel drive type, it will be caused that the wheel speeds of the rear wheels will not become zero, while the wheel speeds of the front wheels will become zero, without the center differential locking mechanism, so that it will be difficult to hold the vehicle in its stopped state at a desired position. Furthermore, in the case where the vehicle is restarted after it was stopped on the rough and steep slope, the braking force will have to be released gradually, to restart the vehicle slowly.